Objective: To determine whether body size and perceived figure, both current and historical, are associated with a diagnosis of endometriosis on laparoscopy. Design: Cohort study of consecutively identified patients undergoing laparoscopy for tubal sterilization or as a diagnostic procedure. Setting: Two university-affiliated hospitals. Patient(s): A cohort of 84 women ages 18-45 years. Endometriosis was visualized in 32 cases; 52 women (controls) had no visualized endometriosis, including 23 undergoing tubal sterilization and 30 with other gynecologic pathology. Main Outcome Measure(s): Body mass index (kg/m2) from self-report and perception of body figure were compared for their ability to predict case status (diagnosed endometriosis), using logistic regression models. Longitudinal trends in BMI based on perceived figure at 5-year intervals from age 15 years were compared using mixed linear models. Results: Based on self-report, women diagnosed with endometriosis were taller, thinner, and had a significantly lower BMI. In this series, cases were more likely to be late maturers (menarche--14 y) and late to initiate sexual activity (21 y), while they were less likely to be gravid, parous, and a current smoker. Adjusting for age (in years), being tall (height--68 in), and parity (yes, no), a higher current BMI was statistically protective for a diagnosis of endometriosis, regardless of whether BMI was determined by self-report (adjusted odds ratio [AOR]=0.88, 95% confidence interval [CI] 0.79-0.99) or from perceived figure (AOR=0.86, 95% CI 0.75-0.99). For every unit increase in BMI (kg/m2), there was an approximate 12-14% decrease in the likelihood of being diagnosed with endometriosis. In an adjusted repeated measures model, BMI was 21.3/0.6 kg/m2 (estimate--SE) for women with endometriosis, compared with 23.2/0.4 kg/m2 for the controls, a difference over all ages of -1.9/0.8 kg/m2 (P = .045). This is a consistent difference of about 10 lb at every age, assuming an average height of about 64.5 in. Conclusions: In a laparoscopy cohort, women diagnosed with endometriosis were found to have a lower BMI (leaner body habitus), both at the time of diagnosis and historically. That women diagnosed with endometriosis may have a consistently lean physique during adolescence and young adulthood lends support to the suggestion of there being an in utero or early childhood origin for endometriosis.